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CIS 460 - Network Analysis and

Design

Chapter 2

Analyzing Technical Goals and

Constraints

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Analyzing Technical Goals

and Constraints Goals include:

scalability

availability

performance

security

manageability

usability

adaptability

affordability

Tradeoffs associated with these goals

Provides terminology to discuss technical goals

with customer

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Scalability

How much growth a network design must support

maybe a prime goal for some

Proposed design should be able to adapt to increase inusage/scope

Planning for Expansion

How many sites to add

How extensive the networks for each new site How many more users to access the corporate network

How many more servers or hosts to add to theinternetwork

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Scalability (Contd)

Expanding the Data Available to Users

Empowered employees make strategic

decisions that require access to sales,marketing, engineering and financial data

The 80/20 rule is no longer valid (80 % stays inlocal LANs, 20% destine for other places)

Increasing access to WWW servers Increasing intranet usage

Strategic alliances with suppliers/customers

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Scalability (Contd) Technical goals for scaling/upgrading enterprise

networks:

Connect separate department LANs in corporateinternetworks

Solve LAN/WAN bottleneck problems Provide centralized servers that reside on server farms

Merge an independent SNA network w/enterprise IPnetwork

Add new sites to support field offices & telecommuters

Add new sites to support communication withcustomers, suppliers, resellers, and other businesspartners

Constraints

There are impediments to scalability due to incorrect

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Availability

Refers to the amount of time a network isavailable to users and is often a critical goal

Can be expressed as a percent uptime per year,

month, week, day, or hour Also lined to reliability but has a more specificmeaning (percent uptime) than reliability.Reliability refers to a variety of issues, includingaccuracy, error rates, stability and TBF.

Also associated with resiliency - how much stressa network can handle and how quickly a networkcan rebound from problem.

Disaster recovery. Disaster recovery plan

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Availability (Contd)

Specifying Availability Requirements

Specify with precision (% uptime), timeframe(day or nightdowntime), time unit (day, week, etc.)

Cost of Downtime

For critical applications document how much lost per hour

Also helps to determine whether in-service upgrades must besupported

MTBF/MTR

Define as mean time between failures and mean time to repair

They can be used to calculate availability goals

4000 hours MTBF is a common goal

Good idea to identify for specific applications in addition tonetwork as a whole

Can usually use data supplied by manufacturer for equipment

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Network Performance

Criteria for accepting network performance

throughput, accuracy, efficiency, delay, and response

time Tightly tied to analyzing the existing network to

determine what changes need to be made to meetperformance goals.

Also tightly linked to scalability goals Definitions

capacity (bandwidth), utilization, optimum utilization,throughput, offered load, accuracy, efficiency, delay(latency), delay variation, response time

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Network Performance

(Contd)

Optimum Network Utilization

measure of how much bandwidth is used during aspecific time period. Commonly specified as apercentage of capacity

Various tools 7used to measure usage and averaging theusage over elapsed time.

Customer may have a network design goal for themaximum average network utilization allowed on

shared segments. For shared Ethernet should not exceed 37 percent else

collision rate becomes excessive

Toke Ring/FDDI typical goal is 70 percent

For WANs optimum is also about 70 percent

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Network Performance

(Contd)

Throughput

the quantity of error -free data that is transmitted perunit of time

Ideally should be the same as capacity, however noutusually

Capacity depends on the physical-layer technologies inuse

Depends on the access method

Throughput of Internetworking Devices

Some specify goals as Number of packets per second

The maximum rate the device can forward packets

without dropping

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Network Performance

(Contd)

Application layer throughput

a measure of good and relevant application-layer datatransmitted per unit of time (also called goodput)

Can increase throughput not goodput because extra datatransmitted is overhead data

Usually measure in kilobytes or megabytes per second

Constraints

end-to-end error rates

protocol functions such as handshaking, windows, &acknowledgments protocol parameters such as frame size

protocol parameters such as frame size and retransmissiontimers

PPS or CPS rate of internetworking devices

Lost packets or cells at internetworking devices Workstation and server performance factors

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Network Performance

(Contd) Accuracy

goal is that data received at the destination must be the

same as the data sent by the source. Typical causesinclude power surges or spikes, failing devices, noise,impedance mismatch, poor physical connection

Accuracy goals can be specified as a bit error rate(BER)

On shared Ethernet errors are often result of collisions Collision that happens beyond the first 64 bits of a

frame is a late collision which are illegal and shouldnever happen

In token rings accuracy goals sometimes include goals

for minimizing media-access control error reports

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Network Performance

(Contd) Efficiency

measurement of how effective an 0operation is in comparison tothe cost in effort, energy, time, or money

Provides a useful way to talk about network performance

Large frame headers are an obvious cause for inefficiency

Goal is to minimize the amount of bandwidth used by headers

Delay and Delay Variation

Users of interactive applications expect minimal delay inreceiving feedback

Multimedia applications require a minimal variation in theamount of delay that packets experience

Telnet protocol applications are also sensitive to delay because

of echo feedback

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Network Performance

(Contd) Causes of Delay

delay is relevant to all data transmission technologies, butparticularly satellite links and long terrest4rial cables. Alsotime required to put digital data on a transmission line. Packet

switching delay, router delay, queuing delay

Delay Variation

Digital voice and video applications are effected with jitter, ordisrupted communications. Can be minimized using a buffer

to minimize jitter Response Time

the network performance goal that users care about most. Theyrecognize small changes in the expected response time. If lessthan 100 ms most users do not notice

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Security

Overall goal is that security problems should not

disrupt the companys ability to conduct business.

Have protection so that business data and otherresources are not lost or damaged

First task is planning which involves analyzing

risks and developing requirements.

Hackers, financial costs, sensitive of customer data

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Security (Contd)

Security Requirements

protect resources from being incapacitated, stolen,

altered, or harmed. Let outsiders access data on public web but not internal

data

authorize/authenticate branch-office users, mobile users& telecommuters

detect intruders physically secure hosts and internetworking devices

protect applications and data from viruses

train network users and network managers on security

risks

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Manageability

Includes the following network management

functions

performance management

fault management

configuration management

security management accounting management

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Usability

Refers to the ease-of-use with which

network users can access the network and

services

some network design components can have

a negative effect on usability

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Affordability

Sometimes called cost-effectiveness

Primary goal is to carry the maximum amount oftraffic for a given financial cost

In a campus network low cost of often the primarygoal

For enterprise networks availability is usuallymore important than low cost

Minimizing hiring, training, and maintainingpersonnel to operate and manage the network is

also an important goal

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Affordability (Contd)

Making Network Design Tradeoffs

Need to know how important affordability is to other

goals Availability often requires Redundant componentswhich raises costs

To implement affordability might mean availabilitymust suffer

Your customer should identify a single driving networkdesign goal

Sometimes making tradeoffs is more complex thanwhat has been described because goals can differ

for various parts of an internetwork

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Technical Goals Checklist

You should use a technical goals checklist

and Table 2-3 to determine if all the

clients technical objectives and concerns

are addressed.